To speed up the process of contacting people who may have been exposed to coronavirus, the Federal Government is asking Australians to download its new COVIDSafe app.

The more people use the app, the message goes, the faster we can slow the spread of the virus and the sooner we can lift restrictions and return to the pub.

"The first job of the COVIDSafe app is to keep you safe," Prime Minister Scott Morrison said in a press conference on Tuesday.

But how does a smartphone app that identifies who you've been near protect you from COVID-19?

Using an app to help disease contact tracing is largely unprecedented, so it's hard to know how effective it will be or even which technology works best.

In Singapore, where its contact tracing app TraceTogether has been operational since late March, there is still only limited understanding of its impact.

And while there are lingering concerns regarding the app's privacy and security, Adam Dunn, who leads biomedical informatics and digital health at the University of Sydney, said questions also remained about the app's accuracy.

Digital tools to enhance contact tracing

When it's operational, COVIDSafe is designed to digitise part of the coronavirus contact-tracing process.

The app uses Bluetooth to record an encrypted ID from every smartphone that comes into range, if it also has the app.

If a person is later diagnosed with COVID-19 and they consent, IDs from the app will be shared to a central server and used by health authorities to track down people who may have been exposed, and help to break the chain of transmission.

If you're diagnosed with COVID-19 today, you're likely to be interviewed by a contact tracer who will ask where you've been and who you've seen.

The current process can be onerous, said Meru Sheel, an infectious diseases epidemiologist at Australian National University.

While she's mindful of some of privacy concerns around the app, she hopes it will help address lapses in memory.

If cases emerge with unknown links, the process of tracking down where it emerged from and who the person may have encountered can be especially difficult — an app could help speed this up.

"It will have an impact on those unknown linked cases," Dr Sheel said.

Not clear how many need to use app

While the app has the potential to support Australia's extensive contact tracing efforts, it's unclear how many people need to first download it, and then use it properly every day.

The Australian Government has aimed for a 40 per cent uptake of its app, without sharing why.

A mathematical model from Oxford University, on the other hand, suggests around 60 per cent of the population need to use it "stop the epidemic".

Yet even with lower usage, it estimated a reduction in the number of coronavirus cases and deaths is likely if combined with an investment in testing.

But according to Dr Dunn, benefits are likely to be small even if the Government's initial target of 40 per cent uptake in the population is met. The app only records a contact if the other person also has the app.

"If 40 per cent of people download the app, and if we're optimistic, half of those people are using it properly at all times — including having it running and open in the foreground on iPhones — then the likelihood of actually identifying a contact in a random encounter is 4 per cent," he said.

"What that means is that fewer than one in 20 potential contacts would be captured by that app."

But Dr Dunn said if 70 per cent of Australians downloaded the app and used it properly at all times, then up to half of all encounters could be captured by the app.

Then there are those who cannot participate: people who have old phone models that can't support COVIDSafe, people who cannot afford a smartphone at all, or overseas traveller and immigrant workers who don't have Australian phone numbers of local app store accounts.

According to an editorial published in the scientific journal Nature last week: "There is scant published evidence on how effective these apps will be at either identifying people who have not been tested or, if widely used, stopping the spread of the disease."

Overcoming technical problems

To be effective, the app also needs to do what it claims to do — collect IDs from apps in its vicinity.

There are questions about whether COVIDSafe works effectively on iPhones (which could be fixed by coming software updates), but also mixed evidence that Bluetooth provides an accurate and useful measurement of distance, which is needed to work out if someone is a close contact.

The Government has declined to share the algorithm that will identify close contacts on the server.

Even the creators of Bluetooth have expressed doubt, recently telling US outlet The Intercept that assessing distance to another Bluetooth device will "vary significantly depending on conditions".

Dinesh Kumar, a biomedical signals expert in the School of Engineering at RMIT, is concerned that the current version of the app could yield 'false positives' due to the inexactness of Bluetooth.

"It goes through walls and it goes through glass," he said of the wireless technology.

This may be a particular issue in denser areas, such as offices and apartment buildings.

While he supports the app, Dr Kumar said it is not a "magic wand".

Unintended behavioural consequences

When measuring the app's effectiveness, Dr Dunn said it was important to look at how it was working in practice and not just at download numbers.

He said some of the communication around COVIDSafe, including the Prime Minister likening it to sunscreen, could lead Australians to mistakenly believe it confers some kind of individual protection.

"It's all about a false sense of security … if we all download the app, then we can go and watch some sports together, or if we all download the app, then I don't need to stand 1.5 metres away from people," Dr Dunn said.

While COVIDSafe may assist contact tracing efforts, he said the Government needed to make clear it was no panacea for controlling a pandemic.

In his view, community anxiety would be reduced if the government shared evidence-based triggers for when we can re-open business, schools and borders, instead of tying such decisions to the app.

Ultimately, a contact tracing app is "part of a bundle" of ways to address the pandemic, according to Dale Fisher, a professor in infectious diseases at the National University of Singapore.

Dr Fisher, who also works with the WHO's Global Outbreak Alert and Response Network, said this bundle must include early detection of cases, testing and quarantining of contacts.

"It's a useful aid, but on its own, it's just one of the many things you have to do," he said.

The ABC has contacted the Department of Health for comment.